The present invention relates to a variable displacement vane pump applicable to, for instance, a hydraulic power steering apparatus for automobiles.
As conventionally known, a variable displacement vane pump is provided with a relief valve in order to prevent hydraulic equipment from suffering from an excessive pressure rise. The relief valve is constructed such that a ball-shaped valve element is held by a valve element retainer and biased toward a valve seat by a coil spring. However, the relief valve has such a problem that when moving to an open position, the valve element and the valve element retainer are vibrated to cause noise.
Japanese Patent Application Unexamined Publication No. 2003-74725 discloses a relief valve for use in a variable displacement vane pump. The relief valve includes a ball retainer as a valve element retainer which is disposed within a valve bore in an inclined state such that an outer circumferential surface of the ball retainer comes into contact with an inner circumferential surface of the valve bore. With this construction, a friction force is produced between the outer circumferential surface of the ball retainer and the inner circumferential surface of the valve bore upon the contact therebetween, thereby suppressing occurrence of noise.